The present invention relates to a current control apparatus for electric power systems such as an inverter, a cycloconverter and the like. More specifically, this invention relates to a current control apparatus in which phase and amplitude errors between an instruction input data for polyphase currents and actual polyphase currents supplied to a load are avoided.
When a variable frequency alternating current (AC) is supplied to a load by means of an electric power system such as an inverter or cycloconverter, phase and amplitude errors between a control target value and an actual value of load currents cannot be avoided, as well known to a person skilled in the art. Such a prior art problem will be discussed in detail referring to FIG. 1.
FIG. 1 shows a current control apparatus for a conventional cycloconverter which supplies three-phase AC currents to a load. In the configuration of FIG. 1, a current amplitude instructor 101 provides an amplitude instruction I1*. A frequency instructor 103 provides a frequency instruction Vf1* having a voltage which corresponds to an output frequency for the load. A designator 100 generates current instructions for a power converter 10 according to the instructions I1* and Vf1*. A coefficient multiplier 105 multiplies the amplitude instruction I1* by a constant K1. A voltage/frequency (V/F) converter 106 is responsive to the voltage of the frequency instruction Vf1* and generates pulses having a frequency f1*.degree.. The frequency f1*.degree. is proportional to a target output frequency of the cycloconverter. A counter 109 counts the pulse of frequency f1*.degree. to generate a digital AC phase signal .theta.I1* whose contents are periodically circulated. Here, the contents of signal .theta.I1* designate the phase angles of currents to be outputted from the cycloconverter. The AC phase signal .theta.I1* is applied to a function circuit 111 and circuit 111 generates three-phase unit current instructions i1U*, i1V* and i1W*. These instructions may be represented as: ##EQU1##
Each of the unit current instructions i1U* to i1W* and output signal K1I1* of the coefficient multiplier 105 is independently multiplied by multipliers 116 to 118, and AC current instructions i1U*, i1V* and i1W* are obtained from multipliers 116, 117 and 118, respectively. Thus, instructions i1U*, i1V* and i1W* may be represented as: ##EQU2##
FIGS. 1A and 1B show typical configurations of function circuit 111. FIG. 1A is directed to a three-phase configuration and FIG. 1B is directed to a two-phase configuration. In FIG. 1A a digital input .theta. designates a specific address of ROMs 1 to 3 each of which stores cosine function values. According to the contents of input .theta., digital function values xD, yD and zD are outputted from ROMs 1, 2 and 3, respectively. These values xD, yD and zD are converted into analog signals cos.theta., cos(.theta.-2.pi./3) and cos(.theta.+2.pi./3) via D/A converters. In FIG. 1B a digital input .theta. designates a specific address of a ROM 4 which stores cosine function values, and also designates a specific address of a ROM 5 which stores sine function values. Then ROMs 4 and 5 output function values xD and zD, respectively, and these values xD and zD are converted into an analog cosine signal and sine signal by D/A converters.
In FIG. 1, power converter 10 including a control circuit 200 and a cycloconverter 500 supplies a load 600 with AC currents i1U to i1W according to the contents of AC current instructions i1U* to i1W*. Comparators 201, 202 and 203 compare the current instructions i1U*, i1V* and i1W* with the detected values (s1U, s1V and s1W) of load currents i1U, i1V and i1W, respectively, and independently produce error signals .epsilon.1U, .epsilon.1V and .epsilon.1W. Amplifiers 207, 208 and 209 amplify the error signals .epsilon.1U, .epsilon.1V and .epsilon.1W to provide AC voltage instructions v1U*, v1V* and v1W*.
The cycloconverter 500 generates three-phase AC voltages V1U, V1V and V1W according to the contents of AC voltage instructions v1U*, v1V* and v1W*. These AC voltages V1U, V1V and V1W are applied to windings U, V and W of a three-phase induction motor or load 600. When voltages V1U to V1W are applied to the load 600, load currents i1U to i1W corresponding to these voltages flow. Load currents i1U, i1V and i1W are detected by current sensors 507, 508 and 509, respectively. Each of the sensors 507 to 509 isolatedly detects the load currents i1U to i1W, and generates AC load current signals s1U to s1W. Signals s1U, s1V and s1W respectively represent the load currents i1U, i1V and i1W, and the sign of each of the signals s1U to s1W is identical with the sign of each of the currents i1U to i1W. Load current signals s1U, s1V and s1W are fed back to the comparators 201, 202 and 203, respectively.
According to a prior art apparatus having such a configuration as mentioned above, AC current instructions i1U* to i1W* outputted from designator 100 are compared with AC load current signals s1U to s1W in a closed control loop; thus, the load currents i1U to i1W of load 600 depend on the value of AC current instructions i1U* to i1W* (here, for brevity's sake, no consideration is given to possible ripples in the output of the cycloconverter).
FIG. 2 illustrates the relations between an input of the designator 100 (instruction vector I1 ) and each of AC current instructions i1U* to i1W*. In this figure the symbols U, V and W denote the geometrical phase positions of windings of the load 600. When actual currents i1U to i1W corresponding to the instructions i1U* to i1W* (Eq.(2)) flow into the windings U to W of load 600, a current vector I1 obtained by composing the vector components of currents i1U to i1W is generated in the load 600. The current vector I1 becomes identical with an instruction vector I1* according to a feedback operation, and the vector I1* is obtained by composing the instructions i1U* to i1W*.
The current vector I1* has a constant amplitude (=I1*) and a variable phase angle .theta.I1* which is defined in reference to the phase axis of winding U. Angle .theta.I1* corresponds to the frequency instruction Vf1*. The rate of change of the phase angle d.theta.I1*/dt, or the angular frequency of .theta.I1*, is constant unless instruction Vf1* is changed. The actuation of load 600 depends on the current vector I1*, and the vector I1* may be reduced to its vector components i1U* to i1W*. This means that the actuation of load 600 can be controlled by current instructions i1U* to i1W*. Thus, the instruction I1* of FIG. 1 designates the amplitude of current vector I1* of FIG. 2, and the phase signal .theta.I1* of FIG. 1 designates the phase angle .theta.I1* of vector I1*. Threephase (polyphase) AC current instructions i1U* to i1W* of Eq.(2) are produced in a manner such that first, angle data .theta.I1* is generated from elements 106 and 109 according to the frequency instruction Vf1*; second, polyphase unit current instructions i1U* to i1W* are generated from function circuit 111 according to the angle data .theta.I1*; and finally the amplitude of said instructions i1U* to i1W* are controlled by multipliers 116 to 118 according to the amplitude data I1*.
The configuration of FIG. 1 has a substantial disadvantage. The current control apparatus of FIG. 1 depends on an AC feedback control system in which AC current instructions i1U* to i1W* and AC load current signals s1U to s1W are respectively compared with one another to generate AC error signals .epsilon.1U to .epsilon.1W. As is known, an AC feedback control system contains a phase delay element. Then, the frequency response of a closed loop of an AC feedback control system shows a level down and a phase delay in a high frequency region with reference to an AC instruction applied to the control system, and such level down and phase delay become more and more conspicuous as the signal frequency becomes higher. Thus, stationary errors of amplitude and phase in the AC feedback control system increase as the frequency designated by the instruction Vf1* increases.
Where a cycloconverter is utilized in the current control system, as shown in FIG. 1, the cycloconverter generates output currents with large ripples, and amplifiers 207 to 209 suffer a disadvantage due to ripples of the output currents. Large ripples induce an unstable feedback operation. Thus, amplifiers 207 to 209 must have filtering elements for effectively stabilizing the current controllability of the cycloconverter. Because of the substantial phase delay and high-cut frequency characteristic of filtering elements, when the output current frequency becomes high, phase and amplitude errors between the AC current instructions and the actual load currents become serious. Such a problem is discussed in detail in a paper of:
NATIONAL CONVENTION RECORDS OF THE INSTITUTE OF ELECTRICAL ENGINEERS OF JAPAN, 1981, No. 563, pp. 685-686.
Currently, an attempt is being made to vector-control the operation of an induction motor by means of a variable frequency power source, e.g., a power converter using a cycloconverter. Such a power converter requires that each of its output currents exactly correspond to a current instruction in both stationary and transient states without any phase and amplitude errors. Accordingly, a special current control apparatus, which is free from phase and amplitude errors between a current instruction and each of the actual polyphase AC currents supplied to a load, is strongly desired.